How can I read from Redis inside a MULTI block in Ruby?

I'm encapsulating a complicated set of Redis commands in a MULTI transaction, but the logic in the transaction depends on values already in Redis. But all reads within a transaction seem to return nil

Here's an example that demonstrates the problem:

[Dev]> $redis.set("foo", "bar")
=> "OK"
[Dev]> $redis.multi{ $redis.set("foo", "baz") if $redis.get("foo") == "bar" }
=> ["bar"]
[Dev]> $redis.get("foo")
=> "bar"

Obviously I want the last return value to be 'baz' – how can I achieve this?